Chemical process and plant

ABSTRACT

A chemical plant for performing a chemical reaction between particles of a material such as lithium metal, and a reagent such as butyl chloride in solution in hexane, in which one reaction product is a solid material, includes a reaction vessel ( 12 ). Several ultrasonic transducers ( 16 ) are attached to a wall of the vessel (12) so as to irradiate ultrasonic waves into the vessel, the vessel being large enough that each transducer irradiates into fluid at least 0.1 m thick, each transducer irradiating no more than 3 W/cm 2 , and the transducers being sufficiently close to each other and the number of transducers being sufficiently high that the poser dissipation within the vessel is at least 10 W/litre but no more than 200 W/litre. The high intensity of ultrasound ensures tat lithium chloride is cleaned off the surface of the lithium metal particles throughout the vessel ( 12 ).

[0001] This invention relates to a method and an apparatus for carryingout chemical reactions that involve a reaction between a material inparticulate form and a liquid, in which a product of the reaction is asolid which tends to form a coating on the particulate material.

[0002] The invention is particularly suitable for reactions involvingalkaline metals, as these metals are highly reactive but are soft and sonot easy to pump. Chemical reactions with such metals may be performedwith the metal in the form of particles in suspension in an inertliquid, the liquid acting as a solvent for a material with which themetal reacts. For example, lithium metal may be reacted with butylchloride in solution in a solvent such as hexane. Formation of a layerof salt on the surface of the metal particles can suppress thisreaction. It is also desirable to ensure that the metal is alwaysexposed, otherwise side reactions may instead occur, for example butylchloride may react with the desired product, butyl lithium.

[0003] According to the present invention there is provided a method ofperforming a chemical reaction between a first material and a reagent,the reaction being carried out between the first material in particulateform and a liquid that comprises the reagent, one reaction product beinga solid material, the method comprising contacting the first materialand the reagent in a plant, the plant comprising at least one reactionvessel with a plurality of ultrasonic transducers attached to a wall ofthe vessel so as to irradiate ultrasonic waves into the vessel, thevessel being large enough that each transducer irradiates into fluid atleast 0.1 m thick, each transducer irradiating no more than 3 W/cm², andthe transducers being sufficiently close to each other and the number oftransducers being sufficiently high that the power dissipation withinthe vessel is at least 10 W/litre but no more than 200 W/litre, stirringthe contents of the reaction vessel, and energising the transducers.

[0004] The values of power given here are those of the electrical powerdelivered to the transducer or the transducers, as this is relativelyeasy to determine. There will inevitably be losses in convertingelectrical to acoustic power, and in transmitting the acoustic powerfrom the transducer into the fluid within the vessel, but these aredifficult to assess accurately. The transducer is typically at least 90%efficient in converting electrical to acoustic power.

[0005] The power radiated by each transducer may for example be in therange 1-2 W/cm². This is a similar ultrasonic power intensity to thatused in ultrasonic cleaning baths, and is above the threshold requiredto achieve cavitation. Although it is possible to achieve higher powers,such as 10 W/cm², the lower intensity ultrasound specified in thepresent invention can propagate much further through a liquid, and thestresses in the transducers are reduced.

[0006] All the transducers may be energized simultaneously, oralternatively groups of transducers may be energized sequentially. Thetransducers may all be energized at the same frequency, for example 20kHz, or alternatively groups of transducers may be energized atdifferent frequencies for example 20 kHz and 40 kHz. The actualfrequency or frequencies of operation are not usually critical, butmight be as high as 140 kHz or even 200 kHz, as such high frequenciestend to reduce the risk of cavitation erosion.

[0007] For the reaction between lithium metal and butyl chloride theparticles of lithium metal are preferably larger than 1 mm in size, forexample between 3 mm and 10 mm, or about 5 mm. Particles of lithiumchloride are formed by the chemical reaction, and are dislodged from thesurface of the lithium particles by the ultrasonic irradiation; theseparticles are typically smaller than 0.1 mm in size. Preferably thevessel has an outlet port through which liquid from the vessel can beextracted, and preferably this outlet port is provided with a coarsemesh or strainer so that the lithium particles are kept in the vessel.

[0008] The invention will now be further and more particularlydescribed, by way of example only, and with reference to theaccompanying drawing which shows a chemical plant for making n-butyllithium.

[0009] N-butyl lithium is made by reacting butyl chloride with lithiummetal, in for example hexane as a solvent for the butyl chloride and inwhich the lithium metal can be suspended. This reaction may be carriedout using very fine lithium metal particles, but the production and useof such fine metal particles is potentially hazardous. Use of largepieces of lithium metal has not been found practical, because lithiumchloride forms a surface layer that suppresses further reaction.

[0010] The chemical plant 10 comprises two substantially identicalreaction vessels 12, 13, each provided with a variable speed multi-stageagitator 14. Each is also provided with a temperature control jacket 15.Each is of stainless-steel, of wall thickness 2 mm, and to the outsideof the wall are attached a large number of transducer modules 16 (forexample between fifty and a hundred; two are shown, not to scale). Eachsuch transducer module 16 consists of a 50 W piezoelectric transducer 16a which resonates at 20 kHz, attached to a conically flared titaniumcoupling block 16 b by which it is connected to the outside of the wall,the wide end of each block 16 b (where it is connected to the wall)being of diameter for example 63 mm, so that each transducer module 16irradiates 50 W over a circle of diameter 63 mm, that is an intensity of1.6 W/cm. The modules 16 form an array covering substantially the entirewall of each vessel 12 or 13. The energy from all the transducers 16 ais dissipated over the entire volume of the vessel 12 or 13, and thenumber of transducer modules 16 is such that the power density ispreferably about 50 or 60 W/litre.

[0011] All the transducers 16 a may, as described, resonate at 20 kHz,but alternatively some of them may instead resonate at a differentfrequency such as 40 kHz. All the transducers 16 a may be energizedsimultaneously, or alternatively groups of adjacent transducers 16 a maybe energized sequentially. For example all the transducers, 16 a on oneside of the vessel may be energized as a common group, and then all thetransducers 16 a on the opposite side.

[0012] Each vessel 12, 13 is also provided with an outlet port 18covered by a coarse mesh so that only particles less than 2 mm acrosscan pass through. An outlet duct 19, in which is a valve 20, an inlinefilter unit 22, a pump 24, and a cut-off valve 26, connects the outletport 18 of the first vessel 12 to an inlet of the second vessel 13. Theoutlet port 18 of the second vessel 13 is similarly provided with anoutlet duct 29 in which is a valve 30, and an inline filter unit 32.Downstream of the pump 24 a recirculation duct 34-with a cut-off valve36 connects the duct 19 to a recirculation inlet of the vessel 12.

[0013] The first vessel 12 is provided with inlets 40, 41 and 42 forhexane, butyl chloride, and-particles of lithium metal respectively, anda reflux condenser 43. It is also provided with a thermometer 44. Thesecond vessel 13 is similarly provided with inlets 40 and 42, for hexaneand lithium metal respectively, a reflux condenser 43, and a thermometer44. In each case the lithium metal is provided in pieces about say 5 mmacross; their exact shape and size are not critical, and the lithium maybe in large-blocks, for example 0.1 m cubes, or rods say 10 mm diameterand 0.1 m long, or cylindrical pieces say 20 mm long cut from a bar ofdiameter 5 mm or 10 mm. Means are also provided (not shown) for purgingthe vessels 12 and 13 with argon.

[0014] In use of the plant 10, the first vessel 12 after being purgedwith argon is charged with hexane and lithium metal particles throughthe inlets 40 and 42, and the agitator 14 is activated to thoroughly mixand circulate the contents of the vessel 12. The vessel 12 is initiallyheated, using the jacket 15, to a temperature of 50° C. Butyl chlorideis then gradually added through the inlet 41. The butyl chloride reactsexothermically with the lithium metal forming the desired product, butyllithium, and also forming lithium chloride. The temperature of thevessel 12 is controlled by controlling the rate at which the butylchloride is added, and if necessary by supplying a coolant to the jacket15. The temperature of the hexane may rise to its boiling point of 69°C., but any hexane which evaporates is returned to the vessel 12 by thecondenser 43. The ultrasonic transducer modules 16 on the wall of thevessel 12 are energized so that the contents are subjected to highintensity ultrasound for example at 60 W/litre, which breaks off smallparticles of lithium chloride from the surface of the lithium metalparticles.

[0015] By opening the valves 20 and 36 (and closing the valve 26) andenergising the pump 24, liquid from the vessel 12 may be recirculatedthrough the filter 22. Lithium metal particles are too large to passthrough the mesh at the outlet port 18; and so remain in the vessel 12.The filter 22 consequently removes lithium chloride, which is in theform of particles typically of size in the range 1 μm-100 μm. By openingthe valves 20 and 26 (and closing the valve 36) and energising the pump24, liquid from the vessel 12 may instead be passed into the secondvessel 13. This would be done after first purging the second vessel 13with argon and charging it with hexane and lithium metal. Consequentlyany unreacted butyl chloride carried into the second vessel 13 isexposed to a large excess of lithium metal, with which it reacts. Thissuppresses the risk of side reactions. The second vessel 13 is alsosubjected to high intensity ultrasound in the same way as is the firstvessel 12.

[0016] Finally the solution of butyl lithium in hexane can be dischargedthrough the outlet duct 29 of the second vessel 13, through the filter32 which removes the particles of lithium chloride, the much largerpieces of lithium metal being trapped by the coarse mesh at the outletport 18.

[0017] It will be appreciated that a chemical reaction plant may differfrom that described above while remaining within the scope of thepresent invention, and that some details will depend upon the chemicalreagents involved. For example in the above reaction an alternativesolvent may be used, such as cyclohexane which boils at 81° C. The plantmay also differ, for example the vessels 12 and 13 might instead be madeof a different material such as polytetrafluoroethylene, and theultrasonic transducer modules 16 might be coupled indirectly to thevessel, being attached to a slightly larger, concentric wall, the spacebetween the inner and outer walls being filled with a coupling liquidsuch as olive oil that has a higher threshold for cavitation than thesolvent such as hexane used in the vessels. The gap between theconcentric walls is preferably a quarter of the wavelength of theultrasound; the oil in the gap helps match the impedance between thetitanium coupling block and the solution in hexane, so that more of theapplied power enters the reacting fluids within the vessel.

1. A method of performing a chemical reaction between a first materialand a reagent, the reaction being carried out between the first materialin particulate form and a liquid that comprises the reagent, onereaction product being a solid material, the method comprisingcontacting the first material and the reagent in a plant, the plantcomprising at least one reaction vessel (12) with a plurality ofultrasonic transducers (16) attached to a wall of the vessel (12) so asto irradiate ultrasonic waves into the vessel, the vessel (12) beinglarge enough that each transducer (16) irradiates into fluid at least0.1 m thick, each transducer (16) irradiating no more than 3 W/cm², andthe transducers (16) being sufficiently close to each other and thenumber of transducers (16) being sufficiently high that the powerdissipation within the vessel (12) is at least 10 W/litre but no morethan 200 W/litre, stirring (14) the contents of the reaction vessel(12), and energising the transducers (16).
 2. A method as claimed inclaim 1 in which the power radiated by each transducer (16) is in therange 1-2 W/cm².
 3. A method as claimed in claim 1 or claim 2 whereinthe transducers (16) are energized simultaneously.
 4. A method asclaimed in claim 1 or claim 2 wherein groups of transducers (16) areenergized sequentially.
 5. A method as claimed in any one of thepreceding claims wherein the or each vessel (12, 13) has an outlet port(18) through which liquid from the vessel (12, 13) can be extracted, andthis outlet port (18) is provided, with a coarse mesh or strainer sothat the particles of the first material are kept in the vessel (12,13).
 6. A method as claimed in any one of the preceding claims whereinthe first material is lithium metal and the reagent comprises butylchloride, wherein the particles of lithium metal are larger than 1 mm insize.
 7. A method as claimed in claim 6 wherein the particles of lithiummetal are of size between 3 mm and 20 mm.